In order to fix the variable-pitch blades onto the rotationally-driving hub body by means of a device offering suitable mechanical strength, particularly to counter the centrifugal tensile forces which are exerted on the blades due to their high speed of rotation, it is known to use twistable straps constituted by high-mechanical-strength fibers, said straps being arranged radially between the hub body and each blade and being rendered integral at their outer end, with respect to the axis of rotation of the rotor, with the strong structure of the blade and at their inner end with the hub body.
Thus, in the patent FR-A-2 430 354, is described a method for constituting a multi-blade propeller comprising at least one pair of opposed blades. This pair of opposed blades is constituted in essence by, on the one hand, a single elongated strap, made in high-mechanical-strength fibers agglomerated by a synthetic resin, said strap being fastened at its middle to a drive hub and, on the other hand, by two aerodynamically-profiled shells fixed respectively to said elongated strap on either side of the hub, the inner end with respect to the axis of rotation of the rotor of each shell being integral with a control member for the pitch of the corresponding blade, produced in such a way as to exert, on said shell, a torsion couple centered on the longitudinal axis of the elongated strap. This strap is formed by two flat half-bundles arranged side by side along the small dimension of their cross section and slightly spaced at their mid-length in such a way as to let the sliding shaft for control of the pitch of the blades pass through.
A propeller can thus be constituted by a plurality of such pairs of opposed blades, the respective strap forming the various twistable tie rods connecting said aerodynamically-profiled shells being superimposed in planes perpendicular to the axis of rotation of said rotor, in the area of their passage close to the control shaft and coated by a binder integrating the set of these straps together in this central area, where they thus constitute the hub body, which is integral with a drive shaft.
Moreover, there is known, by the patent FR-A-2 041 747 a link element, and its manufacturing method, between two rigid bodies, which comprises a bundle of high-mechanical-strength fibers arranged longitudinally in this element. Such a link element, flattened in shape, may constitute the retaining attachment for a rotor blade onto the hub body of said rotor, said bundle being twistable in such a way as to allow the controlled pitch variations of said blade. The strong armature of such a blade may then be constituted by extensions of such bundles into its aerodynamically-shaped part.
Such embodiments however comprise limitations. In the patent FR-A-2 430 354, the blades have to be opposed, thus imposing an even number of blades, which can be troublesome if, for acoustic reasons for example, it is suitable to install an odd number of them. Moreover, the straps are stacked in the hub body and there is thus no rotational symmetry, each strap turning in a plane which is peculiar to it, which dictates that a different torsion is provided for each strap so that the corresponding points on each strap can turn in a common plane. Moreover, the fact that each half-bundle partially surrounds the central bore of the hub body over half of its circumference, implies that these straps occupy a relatively large volume in the central area of the hub body. The half-bundles are then trapped in this area by the binder over a relatively great length and are thus unable to provide any torsional elasticity in this area, in particular for following the controlled pitch variations of the blades. It is then necessary to elongate the twistable part of the straps in order to obtain the torsional elasticity necessary for correct operation of the rotor up to its pitch limits.
In the embodiment described by the patent FR-A-2 041 747, it is possible to have an even or odd number of blades, but the total number of constituent bundles of said blades is limited due to the fact that these bundles of fibers are bent on the flat in the hub body, hence along the direction where they exhibit the greatest inertia, by an angle equal to that existing between two neighboring blades, which entails the existence of substantially different lengths between the fibers of the same bundle and, thus, poor distribution of the mechanical stresses between the fibers of this bundle.